Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography. Flexographic printing plates are relief plates with image elements raised above open areas. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made.
Although photopolymer printing elements are typically used in “flat” sheet form, there are particular applications and advantages to using the printing element in a continuous cylindrical form, as a continuous in-the-round (CITR) photopolymer sleeve. CITR photopolymer sleeves add the benefits of digital imaging, accurate registration, fast mounting, and no plate lift to the flexographic printing process. CITR sleeves have applications in the flexographic printing of continuous designs such as in wallpaper, decoration and gift-wrapping paper, and other continuous designs such as tablecloths, etc. CITR sleeves enable flexographic printing to be more competitive with gravure and offset on print quality.
A typical flexographic printing plate as delivered by its manufacturer, is a multilayered article made of, in order, a backing or support layer, one or more unexposed photocurable layers, a protective layer or slip film, and a cover sheet. A typical CITR photopolymer sleeve generally comprises a sleeve carrier (support layer) and at least one unexposed photocurable layer on top of the support layer.
A flexographic printing element is produced from a photocurable printing blank by imaging the photocurable printing blank to produce a relief image on the surface of the printing element. This is generally accomplished by selectively exposing the photocurable material to actinic radiation, which exposure acts to harden or crosslink the photocurable material in the irradiated areas. The photocurable printing blank contains one or more layers of an uncured photocurable material on a suitable backing layer. The photocurable printing blank can be in the form of a continuous (seamless) sleeve or as a flat, planar plate that is mounted on a carrier sleeve.
The photopolymers used generally contain binders, monomers, photoinitiators, and other performance additives. Photopolymer compositions include those described in U.S. patent application Ser. No. 10/353,446 filed Jan. 29, 2003, the teachings of which are incorporated herein by reference in their entirety. Various photopolymers such as those based on polystyrene-isoprene-styrene, polystyrene-butadiene-styrene, polyurethanes and/or thiolenes as binders are useful. Preferable binders are polystyrene-isoprene-styrene, and polystyrene-butadiene-styrene, especially block co-polymers of the foregoing.
The printing element is selectively exposed to actinic radiation in one of three related ways. In the first alternative, a photographic negative with transparent areas and substantially opaque areas is used to selectively block the transmission of actinic radiation to the printing plate element. In the second alternative, the photopolymer layer is coated with an actinic radiation (substantially) opaque layer that is sensitive to laser ablation. A laser is then used to ablate selected areas of the actinic radiation opaque layer creating an in situ negative. This technique is well-known in the art, and is described for example in U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, and in U.S. Pat. No. 5,925,500 to Yang et al., the subject matter of each of which is herein incorporated by reference in its entirety. In the third alternative, a focused beam of actinic radiation is used to selectively expose the photopolymer. Any of these alternative methods is acceptable, with the criteria being the ability to selectively expose the photopolymer to actinic radiation thereby selectively curing portions of the photopolymer.
Next, the photopolymer layer of the printing element is developed to remove uncured (i.e., non-crosslinked) portions of the photopolymer, without disturbing the cured portions of the photopolymer layer, to produce the relief image. The development step has traditionally been accomplished in a variety of ways, including water washing, solvent washing, and thermal development (blotting). Thermal development has the advantage of not requiring an additional drying step after development and thus provides the ability to go more quickly from plate to press.
Processes have been developed whereby photopolymer printing plates are prepared using heat and the differential melting temperature between cured and uncured photopolymer is used to develop the latent image. The basic parameters of this process are known, as described in U.S. Pat. Nos. 5,279,697, 5,175,072 and 3,264,103, in published U.S. patent publication Nos. US 2003/0180655, and U.S. 2003/0211423, and in WO 01/88615, WO 01/18604, and EP 1239329, the teachings of each of which are incorporated herein by reference in their entirety. These processes allow for the elimination of development solvents and the lengthy plate drying times needed to remove the solvent. The speed and efficiency of these processes allow for their use in the manufacture of flexographic plates for printing newspapers and other publications where quick turnaround times and high productivity are important.
In order for the printing plate to be thermally developable, the composition of the photopolymer must be such that there exists a substantial difference in the melt temperature between the cured and uncured polymer. It is precisely this difference that allows the creation of an image in the photopolymer when heated. The uncured photopolymer (i.e., the portions of the photopolymer not contacted with actinic radiation) melts and/or substantially softens while the cured photopolymer remains solid and intact at the temperature chosen. Thus, the difference in melt temperature allows the uncured photopolymer to be selectively removed thereby creating the desired image.
During thermal development, the printing element is generally heated to a temperature of at least about 70° C. The exact temperature depends upon the properties of the particular photopolymer being used. However, two primary factors should be considered in determining the development temperature:                1. The development temperature is preferably set between the melt temperature of the uncured photopolymer on the low end and the melt temperature of the cured photopolymer on the upper end. This will allow selective removal of the photopolymer, thereby creating the image.        2. The higher the development temperature, the quicker the process time will be. However, the development temperature should not be so high as to exceed the melt temperature of the cured photopolymer or so high that it will degrade the cured photopolymer. The temperature should be sufficient to melt or substantially soften the uncured photopolymer thereby allowing it to be removed.        
Thereafter, uncured photopolymer can be softened and/or melted and removed. In most instances, the heated printing element is contacted with an absorbent material that absorbs or otherwise removes the softened and/or melted uncured photopolymer. This removal process is generally referred to as “blotting”.
Upon completion of the blotting process, the printing plate element may be post-exposed to further actinic radiation and/or subjected to detackification, cooled and is then ready for use.
While various thermal development apparatuses have been developed for use in the production of relief image printing plates, there remains a need in the art for a thermal developing device that can provide more efficient removal of uncured photopolymer. There also remains a need in the art for an improved thermal development process that permit a uniform application of heat and allow the blotting material to more fully remove uncured photopolymer remaining on the imaged and exposed surface of the relief image printing element.
The inventors of the present invention have determined that better contact between the blotting (absorbent) material and an imaged and exposed surface of a relief image printing element can be obtained by backing the blotting material with an endless impression belt so that more of the blotting material contacts the surface of the relief image printing element, thereby effecting better removal of the uncured photopolymer from the imaged and exposed surface of the relief image printing element.